Present day production level uranium enrichment, i.e. separation of the U-235 isotope, is achieved by what is commonly known as the gaseous diffusion technique. According to known aspects of this technique, molecules of uranium hexafluoride (UF.sub.6) are forced under pressure through sets of small holes or channels in a diffusion screen which constricts the flow of the uranium hexafluoride gas and very slightly affects its flow rate in accordance with molecular weight. Thus, the flow rate through the channels will be a function of isotope type, permitting a small, typically a fraction of one percent, enrichment per stage. The enrichment is typically improved by cascading several stages using both feedback and feedforward systems between waste and product streams, respectively.
The very small mass difference between the atoms of different isotope types making up the uranium hexafluoride molecule and even smaller total relative mass difference between the complete molecules places a limit upon the difference in diffusion rates which may be achieved for the isotopically distinct molecules. This limit is, as indicated above, very small and necessitates the cascading of many stages of diffusion channels if uranium is to be enriched from its naturally occurring concentration of about 0.7% up to approximately 2-4% for typical use in power-generating reactors.
Another approach to uranium enrichment is that described in U.S. Pat. No. 3,772,519, which utilizes differences in radiation absorption frequency between isotope types, particularly of elemental uranium, to permit ionization of particles of one isotope type so that a separation may be created electrically.
A different approach to uranium enrichment, also using lasers, but for vibrational excitation as opposed to ionization, is shown in U.S. Pat. No. 3,996,470, issued Dec. 7, 1976 and U.S. Pat. No. 4,039,411 issued Aug. 2, 1977, both assigned to the same assignee as the present application. In these applications, as in the present invention, finely tuned, radiant energy from a laser is applied to diffusing molecules to induce a vibration in the molecules having atoms of a selected isotope type. The isotopically selective vibrational excitation permits more efficient isotope separation than in diffusion techniques generally. More particularly, where diffusion rate is the operative factor to induce separation among the isotope types, the applied radiant energy and resulting vibrational excitation of the gaseous molecules of the selected isotope type can be made to adjust the accommodation coefficient of the molecules flowing through diffusion channels, whereby the molecules of the vibrationally-excited isotope will accommodate or stick less readily to the channel walls, and thereby diffuse at a higher rate than the unexcited isotope. Isotopically selective vibrational excitation has also been described as producing a conversion from vibrational to translational excitation of the selected isotope type molecules by collision with the molecules of an inert carrier or background gas so that separation may be accomplished by exploiting the difference in translation between the molecules of the selected isotope type and of other molecules of the same components but different isotopes.